Polytetrafluoroethylene coating compositions, method of application to substrates, coated substrates, and films



FGLYKETRAFLUORGETHYLENE CQATING CDl /H- MEES MGNS, MIETHUD ti)?APPUCATEGN 5U5- S'E'RA'EEE, QQATED SUBSTRATES, AND FELMS orneilftochberg, Newburg'lt, N. Y., assignor to E. i. do ..."ont dlslenuours and Company. Wilmington, Del.

corporation of Delaware No iirawing. Application December 21, 1953,Serial No. 399,567

18 Claims. (Cl. li7-lll3) loidal polytetrafiuoroethylene in water asdisclosed in S. Patent 2,478,229 to Berry and U. S. Patent 2,534,058 toRenfrew.

it has long been known that polytetrafiuoroethylene films which aredeposited on a substrate by a single application of an aqueoussuspension of polytetraffuoroeth-ylene tends to develop cracks duringthe drying and baking steps when the dry film thickness exceeds theorder of l to 2.5 mils depending upon the particular lot of aqueouspolytetrafiuoroethylene suspensoid used. Each different lot ofsuspensoid possess-:3 its own "critical cracking thickncss value whichis the dry film thickness above which objectionable cracks developduring drying and/or fusing. The cracks formed during drying of thepolytetrafiuoroethylene aqueous suspensoid vary in size from unmagnified visual cracks to microscopic cracks. tends to develop not only infilms of uniform thickness greater than about I to 2.5 mils but alsowhere such a .lm thickness occurs accidentally because of poor controlin application or because of the shape of the object being coated. Suchcracks not only adversely affect the appearance of the article but theylead to premature failure of the coating.

For certain purposes, particularly in the coating of woven. glassfabrics and various metals, it is necessary or desirable to havepolytctrafluoroethylene films greater than 3.5 mils thick and sometimeseven as high as 5 to 10 mils thick. Under very closely controlledconditions, relatively thick crack-free polytetrafiuoroethylene filmscan be built up on a substrate by applying several consecutive thincoats of an aqueous suspension, i. e., of the order of .5 mil or less.each followed by a separate baking or fusing operation after each coat.It is obvious, however, that this method is laborious, cumbersome andcostly; and, furthermore, it does not eliminate the need for extremecare in avoiding heavy beads or fatty edges when coating irregularlyshaped surfaces.

U. S. Patent 2,539,329 which issued January 23, 195i. to P. F. Sandersdiscloses a mechanical method of eliminating cracks inpolytetrafluoroethylene coatings applied from an aqueous suspension toinorganic fabrics. such as, e. g. woven glass fabrics by calendaring,but it is obvious that such a process involvesan additionalmanufacturing operation which increases the cost. Further, the pressuretreatment required to close the cracks tends to crush and break at leastsome of the glass filaments of the woven l 55 fabric which reduces thetensile strength.

The principal object of this invention is to increase the This crackingt critical cracking thickness of a film deposiited from a single coat ofan aqueous polytetrafiuoroethylene coating composition.

Another object of this invention is to provide modified aqueouspolytetraliuoroethyleue coating compositions which yield in one coatcrack-free films substantially.

thicker than those obtainable with polytctrafiuoroethylene alone in onecoat.

A further object is to provide liquid modified polytetrafiuoroethylenecoating compositions which do not crack when applied at dry filmthicknesses above the order of l to 2.5 mils, this thickness beingrepresentative of dry film thickness at which films ofpolytetrafluoroethylene alone can be expected to crack when applied in asingle coat.

A still further object is to provide a liquid modifiedpoyltctralluoroethylcnc coating composition which can be applied to asubstrate in one coat to yield dry crack free protective coating whichis substantially thicker than the thickest dry crack-free film capableof being produced by a particular aqueous polytctrafluorocthylenesuspension, the critical cracking thickness" of which is to be improved.

A still further object is to provide a substrate with thick crack-freemodified polytetrafluoroethylene coatings.

A still further object is to provide a relatively thick crack-freeunsupported film of modified polytetrafiuoroethylene.

A still further object is the provision of a polytetrafiuoroethylenecoating composition with improved adhesion to metal.

These objects are accomplished by incorporating into an aqueousdispersion of colloidal polytetralluoroethylene a substantial amount ofan aqueous solution of an alkali metal silicate and applying theresulting composition to a substrate.

Any aqueous solution of an alkali metal silicate may be used to blendwith the polytetrafluoroethylene aqueous dispersion. The molar ratio ofthe alkali metal oxide to SiOz in the silicate solutions may vary over awide range, c. g. from lzl to 1:4 or higher as long as the silicates arewater soluble. Silicate solutions having such widely varying ratios arecommercially available.

'l'hc term critical cracking thickness-- as used throughout thespecification and claims means the dry film thickness above which cracksdevelop in a film deposited from one coat of an aqueous dispersion ofpolytetralluoroethylone. Throughout the specification and claims theterms dispersionf suspensoid" and suspension are used synonymously todenote a composition which contains discrete particles distributeduniformly throughout a liquid medium. The term crack means a linearrupture or fissure which extends from the outer surface of a filmpartially or completely through the film to the substrate beneath, suchfissure being at least large enough to be visible to the naked eye orwhen magnified one hundred times under good illumination. The termscrack-free" and free of cracks" mean that the film in question containsan average ot not more than one of the above defined cracks for each 100sq. centimeters of film surface area.

A method of determining the critical cracking thickness of apolytctrafluoroethylene suspcnsoid consists of (1) pouring an aqueoussuspensoid of polytetrafiuoroethylene in a container having 11 slightlytiltcd bottom in an amount that when the water is evaporated thethickness of the dry film at one end of the pan will be about .l milthick and about if) mils thick at the opposite end, (2) drying the filmat about lflt)" C. until substantially all of the water has evaporated,(3) baking or fusing at the required temperature and (-l) examining thecoating by the naked eye, or under magnification if necessary, todetermine the greatest thickness at which no cracks are present.

in a copending application, Serial No. 24l.l.70, filed August 9. i951,now U. S. Patent 2.681.324, there is disclosed a method of improving thecritical cracking thickness of aqueous polytetralluoroethylenecompositions by incorporating organic polymeric materials which are filmforming and produce crack-free films thicker than the thickest filmcapable of being produced from an aqueous 1. spensoid of thepolytetrafluoroethylene. The invention described in this applicationdiffers from the earlier mentioned above in that the modifying materialfor etrafinoroethylene is an aqueous solution of an tal siiicate.following detailed description is given by way of illustration and notlimitation. The parts and percentages are on a weight basis.

EXAMPLE l Per cent by wt. Aqueous polytetrafiuoroethylene suspensoid:

Poiytetrafiuoroethylene 46.0 9} 0 Gctyl phenyl polyglycol ether 2.8

Water 51.2

2 7.69? aqueous solution of sodium silicate 9.0

he 37.6% sodium silicate solution contained 10.6% 7 and 2 .0% SiOz,which is equivalent to a molar ratio (NaQO to SiOzl of l to 2.5.

The above composition was prepared by adding the sodium silicatesolution to the polytetrafluoroethylene ensoid with moderate stirring.The above composiwas cast into a film'on a chromium plate at roomtemperature, dried at 230 F. and fused by heating above the fusiontemperature of polytetrafiuoroethylene which corresponds "to 621 F. Acrack-free film was formed h was approximately 25 mils thick. The filmwas rawn to a thickness of 1 mil. The tensile strength .1 samplei"x.001x% registered 2.5 lbs. on at Scott which is equivalent to 10.000lbs. per sq. inch of cro. -sectional area. The dielectric strength ofthe drawn film corresponded to 2500 volts per mil.

EXAMPLE ll A crack-tree film, was prepared from a composition similar tothat described in Example I by casting a film on a polished chromiumplate followed by drying at about 23 F. and then'heating above 62l F. tofuse the polytetrafiuoroethylene. A second coat was cast on the d filmattached to the chromium plate. followed by ing and fu ing under thesame conditions. The fused was tripped from the chromium plate. It had aappea ance. was free of cracks and had an averthickness of 6.2 mils. hedielectric strength of the fused film as stripped from the chromiumplate was 500 volts per mil of'thickness. After cold drawing the film tothree times its original length to a thickness of approximately 2 milsthe average dielectric strength was increased to 1120 volts per mil.

EXAMPLE in The unstretched fused film of Example ll as stripped fr m thechromium plate was subjected to hot working or rolling by passingbetween heated (270 F.) even speed calender rolls, twice in onedirection and then twice in the opposite direction.

The calendering or rolling operation reduced the average thickness ofthe film from 6.2 mils to 3.] mils and the average dielectric strengthwas increased from 500 \Oils per mil to l l90 volts per mil.

EXAMPLE IV standard square-weave glass fabric, identified as Iii) lit)

lib

Owens-Corning Fibcrglas Corporations ECC-llZ and having the followingspecification:

Mil thickness 3.0 Yarn, size 450% Thread count 40x 39 Ounces per sq. yd2.09

was given three dip coats of the following dispersion:

Per centby wt.

The coated fabric was dried by passing through a heated tower in whichthe air temperature was about Z50-300 F. The thickness of the dry coatedglass fabric at this stage was approximately 4.7 mils and there weremicroscopic cracks in the coating. The cracked coating was calendered toclose the cracks and then subjected to an air temperature of 700-800 F.to sinter the coating. The coated fabric was further coated by dippingin the sodium silicate-polytetrafluoroethylene composition of Example 1and then dried and fused by subjecting to an air temperatureof 700-800F. There were no cracks in the dried sodiumsilicatepolytetrafiuoroethylene coating. The dry thickness after thesecond composition was applied corresponded to 6.7 mils.

in this example it is preferred to coat the glass fabric with unmodifiedpolytctrafiuoroethylene before coating with the aqueous sodium silicatecontaining composition, since the latter has a solvent action on theglass filaments. a

The dry film pickup of the second composition amounted to 2 milsthickness. i. e., approximately 1 mil on each side. In anotherexperiment in which the second composition was the same as the first. i.e.. an unmodified aqueous suspensoid of polytctrailuoroethylcne, thepick-up of the dry film was only .2 mil or .1 mil on each side and afterdrying there were cracks in the surface of the coating.

The polytetrafiuoroethylene-sodium silicate coated glass fabric sampleof this example was divided into four portions designated A, B, C and D.The A sample was not processed further and served as the control. The Bsample was given a heat and pressure treatment by passing it four timesbetween heated (270 F.) smooth, even speed. calender rolls underpressure. Sample C was calendered between unheated uneven speed steelrolls operating under pressure, such as used for milling rubber. SampleD was cold worked on both sides by scraping each side with a knife bladeWhile the sample was supported on a hard smooth surface. After the mechanical surface treatment of samples 8, C and D they were rendered moretransparent. Each sample was then tested for dielectric strength withthe following results:

The above data indicate that hot or cold working of the coatingincreases the dielectric strength.

The coated glass fabric sample of this example showed no loss in weightafter soaking three weeks in water. The coating was not affected in anymanner as a result of the three week water soak. This was surprisingsince the sodium silicate introduced into the coating composition iswater sensitive.

5 EXAMPLE V A crackdree unsupported film 4.0 mils thick was pared fromthe following composition in a single coat:

For cent by wt.

entire surface of each copper and steel. panel, coating both the primedand unprirned surface:

Per cent by wt. Polytetrafluoroethylcne susgacnsoid (some as usedAqueous polytetrafiuoroethylene susponsoldz l Compobmon)Polytotrailuoroethylene d. 46.0 g cm pbenyl poly/glycol Ether 2 8 3.0sodium salt or a rncnure or long .iain alconols, watp'r A Mpredominantly lauryl alcohol u 1.9

- I Non-ion c dis-lensing o ent 1.3

17.-* a ueo s solution 0 sodium silicate 35.0 i b r r q u L 10 Toluene10,9

4 Polyethylene glycol other of an nllryinterl phenol.

line above composition was thoroughly mixed and 1 H 03 a Chmmium g Thefil was (Mad at 230 .Eaon successive coat of the above composition wasF. and then further heated to at least 62l F. to sinler Cured y heatingf" 5 l fli 250 F. The resisthp fig "[1 d m was m' f 1 once of the curednlms to stripping Irorn the copper and Chmmium plate h was i]k h andTcndercd steel panels was measured by 21 Scott tensile tester. The

x n I, r. more transparent when the surface was cold worked by rccoldt'd*JMQW m Ldble Tai ie 3 Pounds ltvquirt-d to Pull o1" Strip From Panel II Ratio of Polytotralluosrtfi thyl-l l l I l t Potass'un i eat l idrrimeruniufl. 01 9 1 sax/10.7 Ion 23.4 l 11 4 25.6 ti6.7, 33.3 mas 31.5sun/41.2 c 0.s* 1.0 3.5a 3.3} 3.31 2.5 2.8

1 Control no primer.

romping with a knife blade when supported by a hard H] "Ehc examplesillustrate blending the aqueous sodium Suriaca or potassium silicatesolutions with the polytetrarluoro- EXAMPLE Vi ethylene. It is to beunderstood that it is within the x fii 3 0 i1 2, was m scope of thisinvention to use a mixture of s dium sili- A f uxnsup?9nec m i A 5;:cats and potassium silicate solutions for modifying the Band we wnowmgcomgmsmon m a critical cracking thickness of the polytctralluoroelhylencPer cent by wt. aqueous dispersions.

Aqueous polytetrafluoroethylene susponsoid: 1 Additional tests on thecoatings and films derivcd from Polytcirallnoroethylene 46-51 90 thocompositions of this invention show that the cracL- Uctyl phcnylpolyglycol ethcr 2.3 i" free coatings also possess ll'c dcsirablopropcrlics of coal- Wgter M 50.7} v lugs dcrivcd frompolytctrulluorocthylcno alone, namely,

IZill /Q aqueous solution of potassium SiilCfl'tC 10 good olcclriculpropcrlics, chemical and solvent resistonce, corrosion rcsistzmcc, andrcsisiuncc to the adhesion 160 or sticking of extrinsic substunccs:lpplicd thereto. Q P 't n comaned in the preferred examples thocoatings are applied by f? T y gi??? 6 9 Co n milky} dipping thesubstrate to be coated in tho cornposilion or I a u r t l Z'Lb/O 2 p bycasting the composition on the surface by well lznown was thorougnlymixed and cast on a chromium platc. h v v I I 5 "r mm (mm at about F andthen further rec niques. in ad 1.1011, the coatings may be upplienhealed to at least 621 F. to sintcr or fuse the film. 'luc by rdocwr(1061013 436 migkywmte film was Stripped from the Chmmium other wellnnown tccnniqucs in the urt of couting surolztte. it was crack-free. 3.0mils thick and the diclecn films with llquld -g COIllPCtSlUOnS- Th6(303151135 {m giy-entrf was 200 l per mil thickness By d may be appliedto one or both surfaces of the substrate "5 the film down to 1.5 milsthickness the dielectric i b8 o ted.

was increased lo 1000 volts pcr mil thickness. An important advantage ofthc products of this invenre tr-ns arenc of the milk -while film wasincreased tion is an economic one. The alkali metal silicate costs 1 Q PY Y I I 14 lli "-3 but a small fraction 01 the cost orpolytetralluorocthylcnc EXAMPLE Vii and provides a blending materialwhich does not detract gteel and comer 9511615 X were prepared f frommany of the desirable properties or" polyletrniluoro coating by sandingwith #240 sandpaper. Each primer ly I I composition illustrated in Table2 below was sprayed my [bus ll. 15 seen that the problem of cracking ofreluon one-half of the prepared surface of a copper and steel livelythick polytctruiluoroeihylcne coatings deposited panel and cured for 3minutes at 750 F. from aqueous dispersions which heretofore hasrestricted Table 2 6 Composition of lrlmt-r P I I l t. "ilu roethyltncl: lotls ium i i l i l A .lo o'mrno" '--r ;.s 3

$216.. if ..i no 1; Mil/Hi7 70. [211 1 71. 1/23.; l3ti.T'll2$.Il (on, i,5 l {trill/41,1: lolytetrslluurot-thylcnc Sus'pcnwul l l l 1 1 pt- "ant.2320 1 09.0 i 59.5 3 f" 8 47.2 i 12.8 l 38 27% PotassiumSllicutcSolulion... ...(lo t 18.0 I 3L0 i 40.5 i 4 z I 52.8 57.2 G1

The polytetrafluorocthylcnc susut-nsnitl consistctl of long cholnalcohols,predominantly luurylulc lolund 37% unti The potassium silicatesolution u'us u 27, corresponds to 7.7% K20 and 19.3%, $101 on a. weightbu. s.

coats of the following poiytetrafluoroethylcne suspensoid coatingcomposition were sprayed over the of polyll-Lruiluoroctllylenc, 3C}, oftho sodium still of a znlrrturu oi aqueous solution in which the molarrutio oi K10 to rl'lol wus 1 tolil l, which finishes, has been solved inan u expected way by modiamazes fying the aqueous dispersion ofpolytetrafiuoroethylertc with another chemically dissimilar material.

In the examples the ratio of dry sodium or potassium silicate to drypolytetrafiuoroethylene varies between about 6 to 94 and 41 to 59. It isto be emphasized that these ratios illustrate the preferred embodiment.In certain applications where film strength is not important. the ratioof silicate to polytetrafiuoroethylene may be as high as 50 to 50, anduseful products may be produced with less silicate than the preferredranges.

The polytetrafiuoroethylene dispersion and/or the mixfare of thesilicate solution and polytetrafiuoroethylene dispersion may containmodifying agents, such as pigments. dyes, soluble chemical substances,inhibitors, dispersing agents, and other modifiers well known in thecoating composition art, to color, stabilize or otherwise modify thechemical or physical properties of the codispersions or the filmsderived therefrom, provided any such modifying agent employed isinnocuous to the comosition and its components.

The products of this invention are particularly useful in coating heatresistant surfaces. such as. metals, ceramics, glass fabrics, asbestosfabrics, woven wire fabrics and heat treated polyacrylonitrile fabrics.The compositions of this invention adhere to copper better than theaqueous dispersion of polytetrafiuoroethylene alone. The coatingcompositions of this invention are also useful for coating substrateswhich have a decomposition temperature below the fusion temperature ofpolytetrafiuoroethylene. such as, cotton, nylon, rayon, and varioussynthetic resins, in which case the baking temperature must be below thefusion temperature of the substrate.

Specific uses for the products of this invention include, in addition tocoated glass fabrics and unsupported films described in the specificexamples, anti-sticking coatings for muffin tins or cookie sheets,electrically insulated coatings for wire, spark plugs, condensers andcorrosion istant interior coatings for metal tanks containing corrosivechemical materi l in coating wire and other metal surfaces with thecrmipositions of this invention, where the maximum adhesion of thecoating to metal is desired, the metal surface may be primed with thepolytetrailuoroethylene-chromic acid compositions disclosed in U. S.Patent 2,562,117 or the polytetrailuoroethylene-chromic acid-phosphoricacid compositions disclosed in U. S. Patent 2,562,118.

it is apparent that many widely different embodiments of this inventioncan be made without departing from the spirit and scope thereof and,therefore, it is not intended to be limited except as defined in theappended claims.

1 claim:

l. A. liquid coating composition comprising an aqueous dispersion ofpolytetrafluoroethylene and an aqueous soluof at least one water-solublealkali metal silicate from the group consisting of sodium silicate andpotassium silicate, said alkali metal silicate being present in anamount of about 6.0% to 50.0% of the combined weight ofpolytetrafiuoroethylene and said alkali metal silicate, the molar ratioof alkali metal oxide to silicon dioxide in the alkali metal silicatebeing at least 1:1.

2. The product of claim 1 in which the alkali metal silicate is sodiumsilicate.

3. The product of claim 1 in which the alkali metal silicate ispotassium silicate.

t. A substrate having a surface coating comprisingpolytetrafiuoroethylene and at least one water-soluble alkali metalsilicate from the group consisting of sodium silicate and potassiumsilicate, said alkali metal silicate being present in an amount of about6.0% to 50.0% of the combined weight of polytetratiuoroethylene and saidalkali metal silicate, the molar ratio of alkali metal 1 to silicondioxide in the alkali metal silicate being at lzl.

5. The product of claim 4 in which the glass fabric.

6. The product of claim 4 in which the metal.

7. The product of claim 4 in which the copper.

8. The process of preparing crack-free polytetrafiuoroethylene filmswhich comprises blending an aqueous solution of at least onewater-soluble alkali metal silicate l'rom the group consisting of.sodium silicate and potassium silicate with an aqueous dispersion ofpolytetrafiuoroet'nylene, spreading the resulting blend in the form of afilm on a substrate, heating to remove the aqueous medium, furtherheating above 621 F. to sinter the film, cooling the sintered film andstripping the sintered film from said substrate. said alkali metalsilicate being present in an amount of about 6.0% to 50.0% of thecombined weight of polytctratluoroethylene and said alkali metalsilicate, the molar ratio of alkali metal oxide to silicon dioxide inthe alkali metal silicate being at least lzl.

9. The process of claim 8 in which the alkali metal silicate is sodiumsilicate.

10. The process of claim 8 silicate is potassium silicate.

I l. The process of preparing crack-free polytetrafiuoroethylenecoatings on a substrate which comprises blending an aqueous dispersionof polytetrafiuoroethylene with an aqueous solution of at least onewater-soluble alkali metal silicate from the group consisting of sodiumsilicate and potassium silicate, applying said blend to a substrate,heating to remove the aqueous medium, further heating above 621 F. tosinter the coating, and cooling the sintered coating, said alkali metalsilicate being present in an amount of about 6.0% to about 50.0% of thecombined weight of polytetrafluoroethylene and said alkali metalsilicate, the molar ratio of alkali metal oxide to silicon dioxide inthe alkali metal silicate being at least 1:1.

12. The process of claim ll in which the alkali metal silicate is sodiumsilicate.

13. The process of claim ll in which the alkali metal silicate ispotassium silicate.

l4. The process of claim ll in which the substrate is glass fabric.

15. The process of claim 11 in which the substrate is metal.

l6. The process of claim ll in which the substrate is copper.

17. A film comprising polytetrafiuoroethylene and uniformly distributedthroughout at least one water-soluble alkali metal silicate from thegroup consisting of sodium silicate and potassium silicate, said alkalimetal silicate being present in an amount of about 6.0% to 50.0% of thecombined weight of polytetrafiuoroethylene and alkali metal silicate.the molar ratio of alkali metal oxide to silicon dioxide in the alkalimetal silicate being at least 1:1.

18. The process of preparing crack-free shaped articles which comprisesblending an aqueous dispersion of polytetratluoroethylene with anaqueous solution of at least one water-soluble alkali metal silicatefrom the group consisting of sodium silicate and potassium silicate,shaping the blend, heating to remove the aqueous, medium, furtherheating to at least 621 F. to sinter the composition and cooling thesintered composition, said alkali metal silicate being present in anamount of about 6.0% to 50.0% of the combined weight ofpolytctrafiuoroethylene and alkali metal silicate, the molar ration ofalkali metal oxide to silicon dioxide in the alkali metal silicate beingat least lzl.

substrate is substrate is substrate is in which the alkali metal SandersAug. 29, i950 Skeda Apr. 3, 1952

4. A SUBSTRATE HAVING A SURFACE COATING COMPRISINGPOLYETRAFLUOROETHYLENE AND AT LEAST ONE WATER-SOLUBLE ALKALI METALSILICATE FROM THE GROUP CONSISTING OF SODIUM SILICATE AND POTASSIUMSILICATE, SAID ALKALI METAL SILICATE BEING PRESENT IN AN AMOUNT OF ABOUT6.0% TO 50.0% OF THE COMBINED WEIGHT OF POLYTETRAFLUOROETHYLENE AND SAIDALKALI METAL SILICATE, THE MOLAR RATIO OF ALKALI METAL OXIDE TO SILICONDIOXIDE IN THE ALKALI METAL SILICATE BEING AT LEAST 1:1.